Velocity reversals via bifurcation in thermal convection

• We have studied numerically bifurcations in thermal convection.
• Comparison with previous experimental results and their explanation has been made.
• The results can be relevant for the development of efficient heat exchangers.

This numerical study reveals and explains a mechanism of eddy emergence in the steady two-dimensional thermal convection. A thin horizontal container is filled with water. The bottom and top walls are adiabatic while the sidewalls have prescribed temperatures. Gravity and a horizontal gradient of temperature drive the water circulation from the cold end to the hot end near the bottom and back near the top. As the flow strength, characterized by the Grashof number Gr, increases, the horizontal velocity reverses and local circulation cells emerge via bifurcation near the central stagnation point. We argue that the reversals are likely caused by the entrainment effect of jets, which form near the horizontal walls. This explains the experimental observations of Kirdyashkin (1984). No instability develops for Gr ⩽ 107 due to the stable vertical stratification of density. The obtained results are of fundamental interest and can be relevant for the development of efficient heat exchangers.